1. Field of the Invention
This invention relates to the production of tripropylene glycol (TPG) and, more particularly, to the production of acrylate grade TPG.
2. Description of the Prior Art
Acrylates, including methacrylates, are valuable chemical building blocks that find many commercial applications such as surface coatings, adhesives, leather and textile finishing, paper coating, and the like. Many commercial acrylate uses require good clarity (transparency) of the final article, e.g., transparent containers.
Urethanes also have valuable chemical uses in the formation of flexible foams, rigid foams, and laminates. Polyurethanes have a wide range of commercial uses such as soft cushioning applications in furniture, vehicles, and the like; laminate padding in clothing; rigid foam insulation; and finishes/varnishes, particularly for surfaces that undergo abusive and/or abrasive wear.
Producers of acrylates and urethanes that go into the making of polyacrylates and polyurethanes often use TPG in their manufacturing processes.
TPG is typically made by hydrolyzing propylene oxide in known manner, but, in so doing, monopropylene glycol (MPG), dipropylene glycol (DPG), tetra propylene glycol (TTPG) and heavier glycols; and one or more aldehydes, including propionaldehyde and aldehydes derived from DPG, TTPG, and the like, are formed in admixture with the desired TPG. Essentially pure TPG must be extracted from this mixture before it can be used in the commercial manufacture of acrylates and urethanes.
Aldehydes mentioned here in above tend to be carried along with the TPG into the acrylate and urethane manufacturing processes. Such aldehydes can have an adverse affect on the clarity of acrylate products that is not tolerable in certain commercial applications. Urethane manufacturing requirements as to the aldehyde content of the TPG used in such manufacture is not as rigid as acrylate manufacturing requirements since, for example, polyurethane product clarity is not important in most polyurethane uses. Therefore, TPG used in urethane manufacturing can have a larger aldehyde content than TPG used in acrylate manufacturing.
Accordingly, there is what is known in the art as acrylate grade TPG and urethane grade TPG. At present, the dividing line between the two grades of TPG is 20 parts per million (ppm) of total aldehyde content in the TPG in question. Acrylate grade TPG can contain no more than 20 ppm of total aldehyde content. Urethane grade TPG can contain more than 20 ppm of total aldehyde content. Both ppm contents are reported as ppm “CHO.”
Therefore, it is important to control the aldehyde content of the final TPG product when the TPG is produced for use in the manufacture of acrylates and urethanes, and particularly acrylates.
The aldehyde content of a TPG product is typically controlled by the use of at least one additive (agent) which reacts with and neutralizes most, if not all, of the aldehyde present in that product. Such aldehyde controlling (reducing) additives are well known in the art and include alkali borohydrides, particularly sodium borohydride. The borohydride, at present, reduces the aldehyde content of TPG to the lowest levels achievable. By use of such additives, the aldehyde content is substantially reduced, if not eliminated, but the additive itself can be left in the TPG product.
Depending on its individual processes and desires, an acrylate manufacturer may be willing to use a TPG product that contains such aldehyde controlling additives, or may, on the other hand, require that the TPG product it buys be essentially free of such additives. In either case, to be acrylate grade, the TPG product must contain no significant amount, no more than 20 ppm, total aldehyde, and often no more than 10 ppm aldehyde.
Accordingly, it is desirable to be able to provide an acrylate grade TPG product either with or without the presence of an aldehyde controlling additive or additives. This invention does just that, and does so with significant cost savings over the practice of the prior art.
The prior art practice, which will be discussed in greater detail here-in-after, has been to make in a single TPG product that is essentially free of aldehyde controlling additive. This has necessitated the transport of large volumes of TPG to a final processor (toiler) which, for a fee, separates out an essentially pure, single, final TPG product that contains essentially no significant amount, as defined above, of such additive.